The septal pore apparatus in intercellular hyphae of Ustacystis waldsteiniae was analyzed by serialsection electron microscopy using chemically fixed and high pressure frozen/freeze substituted samples. Septa have a central "simple" pore with rounded, nonswollen margins. However, the pore apparatus is a highly complex structure, differing significantly from that found in "simple"-and complex-septate basidiomycetes. Three features are typical: 1) the pore canal is traversed by two symmetrically arranged bands, 2) a dome shaped membranous cap, intimately attached to the plasma membrane, encloses the pore on both sides, and 3) a thin electron transparent layer and a layer ofmedium electron density cover the cytoplasmic side of the membranous cap. From our observations of septal pore morphology we suggest a general absence of transport of organelles including ribosomes through the pore.